The invention as herein disclosed relates to the art of extrusion and extrusion lubrication where a metal billet is forced by the application of high pressure through an orifice of cross-sectional area less than that of the original billet. The invention provides a means of reducing the required capacity of the high-pressure source by eliminating or substantially reducing the applied pressure required to overcome container wall friction obtained in a normal or conventional extrusion process. The invention also has the advantage of providing hydrostatic or quasi-hydrostatic extrusion conditions without the attendant complexities of equipment and processes known in the prior art.
Of significant importance in the prior art is the concept known as hydrostatic extrusion where a pressurized fluid medium exerts the force that extrudes the billet through the die orifice. The advancing ram has the function of maintaining a high pressure within the fluid rather than advancing the billet by bearing on the billet end opposite the extrusion orifice. As currently practiced by those skilled in the art, considerable extrusion cycle time is lost to fluid handling, i.e., fluid injection, compression, decompression, and ejection. This necessarily results in production rates (extrusions per unit time) significantly lower than those normally achieved by conventional extrusion methods.
One of the advantages of the present invention is to allow production rates equal or close to those achieved by conventional extrusion in either hydraulic or mechanical presses while still retaining the well-known advantages of plain hydrostatic extrusion.
Also of importance in the prior art is the concept of billet-augmented hydrostatic extrusion wherein the advancing ram maintains contact with the billet during extrusion hereinafter called billet-augmented extrusion. In the method of U.S. Pat. No. 3,382,691, Green, the main purpose of the billet-augmentation pressure is to maintain better billet control than that obtained in plain hydrostatic extrusion, particularly during a condition known as stick-slip where the billet moves intermittently rather than at a constant speed. The billet-augmentation pressure exerted by the ram is carefully controlled in relation to the fluid pressure so that the billet does not plastically upset or bulge into the substantial annular gap maintained between the billet and container. To maintain constant fluid and billet-augmentation pressure levels as the billet extrudes, the excess fluid volume in the billet container is bled off continuously back into a separate hydrostatic pressure container via an axial hole in the extrusion ram. The equipment is necessarily more complex and costly than that for plain hydrostatic extrusion at a comparable fluid pressure level. Also, the production rate are relatively low because of the fluid-handling problem previously cited for plain hydrostatic extrusion.
It is the object of the present invention to improve the hydrostatic extrusion process by using special techniques and procedures which simplify the operation greatly over that currently practiced by those skilled in the art.
The process according to the present invention consists of precoating the billet with two lubrication systems. Each lubrication system may consist of one substance, or several substances together, which perform the intended function. A solid-lubrication system adherent to the billet functions primarily as a die lubricant. A liquid-lubrication system overlays the solid lubrication system and functions primarily as the container lubricant, i.e., in the same way as the hydrostatic medium functions in plain hydrostatic extrusion or billet-augmented hydrostatic extrusion. The liquid lubrication layer is pressurized by the advance of a sealed ram. The presence of a thin film of pressurized liquid lubricant between and the billet and the container wall substantially reduces or virtually eliminates container wall friction during billet extrusion. The radial clearance between the billet and container is preferably kept to the minimum needed to still minimize container wall friction by the presence of a thin pressurized liquid film. Also, only the minimum amount of liquid lubricant is used between the billet and ram at the start in order to allow enough liquid pressurization prior to extrusion to minimize friction. By keeping the volume of liquid lubrication to a minimum, the billets can be precoated with this system by various methods. Precoating of the billets with, in effect, the hydrostatic medium greatly increases the production rate capability of the hydrostatic extrusion process as currently practiced by those skilled in the art.
Another feature of the invention is that the ram is allowed to contact the billet just prior to and during extrusion. This aspect is similar to billet-augmented hydrostatic extrusion. However, by keeping the radial clearance between the billet and container to a minimum and with proper lubrication systems and tooling design, it is possible to achieve the benefits of billet-augmented hydrostatic extrusion without the attendant disadvantages of complex and costly equipment. The preferred radial clearance utilized with the present invention is somewhat dependent on the billet size. For billets of diameters between 1.0 and 4.0 inches this clearance can be approximately 0.010 inch. Smaller diameter billets may utilize a clearance less than 0.010 inch down to a minimum clearance of a few mils. With the exception of these smaller billets the clearance is normally less than 1 percent of the nominal diameter of the billet. In the present invention, the billet may upset but it cannot excessively because of the small radial clearance that is used. Also, the small volume of liquid lubrication system used eliminates the need for bleeding off the liquid lubricant during billet extrusion, as disclosed by the previously cited Green patent concerning billet-augmented hydrostatic extrusion.
Thus, while the preferred range of radial clearance is normally not greater than about 1 percent of the nominal diameter of the billet, larger radial clearances up to about 2 percent are also useful. However, as the clearance is increased much beyond this level, all the problems attendant to handling and pressurizing larger amounts of fluid increase. Among these problems are less billet control because of greater stored energy in the fluid, lower production rates, greater hazards of fire and air pollution when fluids are used at elevated temperature, and higher fluid expense. In addition, larger radial clearances lower the billet volume for a given container size, and this increases the extrusion cost per unit volume of product produced for each push.